| Résumé | sym-Tetra-tert-butylethane, 1, and sym-tetra(trimethylsilyl)ethane, 2, have been studied by 1H, 13C, and 29Si NMR spectroscopy. Both compounds adopt a gauche conformation at room temperature with the Me3C (or Me3Si) groups appearing as two magnetically inequivalent pairs. This conformational preference is attributed to distortion of the tertiary carbons from tetrahedral geometry because of steric repulsion between the geminal alkyl groups. The rotation of one pair of Me3C in 1 can be hindered at low temperatures, but irreversible decomposition sets in at a temperature (140°C) below that required to make the two pairs of Me3C equivalent. However, with compound 2 the two pairs of Me3Si groups can be made equivalent by raising the temperature. It is suggested that equivalence is achieved by a gauche ⇌ anti ⇌ gauche interconversion rather than by a direct gauche ⇌ gauche process. The rate constant can be represented by log (k, s-1) = (8 ± 3) - (10.2 ± 0.4 kcal/ mol)/2.3RT. The large, negative, activation entropy for Me3Si exchange (ΔS≠ = -26 ± 13 gibbs/mol) is attributed to the requirement for a cooperative movement among many groups. Values of T1 for 1 suggest that the tertiary C-H bonds in this molecule are of normal length. Problems associated with measuring the lifetime of the free radical derived by removal of the tertiary hydrogen from 2 are also considered. |
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